Frequency shift tuning indicator system



Patented Aug. 2, 1949 FREQUENCY SHIFT TUNING INDICATOR SYSTEM Reynold S. Chapin, Flushing, N. Y., assignor to Press Wireless, Inc., New York, N. Y., a corporation of Delaware Application September 12, 1945, Serial No. 615.838

12 Claims.

1 This invention relates to wave signalling systems, and more particularly to improvements in signalling systems of the frequency-shift category.

2 tems are provided with automatic frequency control or tuning indicators to keep the apparatus in the in-tune state, separate control channels usually regulated by respective potentiometers A principal object of the invention is to pro- 5 are required for the two conditions correspondvide an improved tuning indicator arrangement ing respectively to mark and space. This has for use in controlling the in tune condition been necessary heretofore, because 0f the freof frequency-shift telegraph apparatus. quency versus voltage characteristic of the usual A feature of the invention relates to a discrimdiscriminators. Such discriminators have only inator and detector combination having the corna single crossover pOint With the ZerO Output aXiS- ponents so correlated and connected as to proregardless of whether the system is in tune or out vide a characteristic having a plurality of crossof tune, and this crossover point corresponds to over points with the zero potential axis. the meanfrequency of the limits between which Another feature relates to a discriminator and the carrier is shifted for mark and space. When detector network having an input frequency verthe frequency of the carrier drifts, this in manisus output voltage characteristic which possesses fested by opposite polarity conditions at the limits spaced parallel linear sloped sections and proof the frequency shift spectrum and the change vides three symmetrical cross-over points with in detected output is greater at one end than at the zero potential axis, the other. Since the transmitted wave may rest A further feature relates to a frequency dison either the mark or space condition for indeficriminator and detector network for use with nite time intervals, any tuning indicator or autofrequency shifted carrier, wherein any out of tune matic frequency control device must respondl or undesired frequency shift condition in the sysequally to either condition. The present invention tem results in potential changes of substantially provides a discriminator and a detector arrangethe same magnitude at both ends of the frement having the necessary equal change output quency-shift range. at the mark and space frequencies regardless of A further feature relates to a discriminator the amount of frequency drift or out-of-tune and detector combination for providing a sawcondition. In accordance with the present intooth overall relation between input frequency vention, the discriminator and rectier combiand output potential. nation has an overall characteristic in which A still further feature relates to the novel orthere are two points of crossover with the zero ganization, arrangement and relative interconvoltage axis e. g., in the nature of a saw-tooth nection of parts which cooperate to provide an characteristic, and which provide output potenimproved frequency discriminating and detector tials varying by the same amount and polarity network for tuning indicator control in frefor a given change in tuning of the receving quency-shift signalling apparatus. apparatus.

Other features and advantages not particu- Referring to Fig. 1, there is represented by the larly set forth will be apparent after a considblock I any well-known source of frequencyeration of the following detailed descriptions and shifted carrier waves. For example, the carrier the appended claims. may have a mean frequency of Fo and it may In the drawing which shows certain preferred be shifted in one direction`to a frequency Fm to embodiments, represent a mark signal, and in the opposite di- Fig. 1 shows ln schematic form a frequencyrection to a frequency Fs to represent a space shift control and tuning indicator arrangement signal, For a, detailed description of a typical embodying the inventive features. carrier frequency-shifting arrangement. refer- Fig. 2 is a schematic diagram 0f a modification ence may be had to Patent No. 2,387,098, filed of Fig. 1. August 12, 1943. The frequency-shifted carrier Figs. 2A to 2D are curves explanatory of the is transmitted over any suitable communication invention. channel 2, either wire or radio, and is received Within recent years.' the SO-called frequenCY 50 in any well-known radio receiver 3 if transmitted shift method of transmitting telegraph mark and by radio. If desired, the mark and space signal space signals has come into comparatively wide may be transmitted as frequency modulations of use. In the case of radio carrier shift transmisa sub-carrier which can be transmitted over the sion, the radiated wave consists of a mark frechannel 2 by amplitude or frequency modulation quency and a space frequency. When such sysof a radio carrier. These mark and space frequencies are detected in the receiver 3 and are applied to any well-known discriminator network 4, which converts the two frequencies into corresponding variable amplitude signals. Preferably, the discriminator 4 is of the balanced type such as disclosed in Radio Engineers Handbook, Terman, section '7, paragraph i8, 1st edition, 1943, pages SBS-586, and has a typical characteristic curve which is represented in Fig. 2h. This characteristic represents a linear change in output voltage with frequency change over the frequency range between the positive and negative peaks.

In Fig. 2A, the mark frequency is represented by Fm and the space frequency is represented by Fs. It will be seen that the mark frequency 1s represented by a positive potential, and the space frequency by a negative potential. Furthermore, if the transmitted mean frequency Fn should change or if the tuned condition of the receiver 3 should change, then the original mark and space frequencies would be represented by respective voltages of different magnitude. In order to avoid these different polarities and possible magnitude differences, the present invention contemplates, in effect, the separation of the positive and negative portions of the usual balanced discriminator characteristic (Fig. 2A) above and below the mean frequency Fo. These separate portions are shifted along the voltage axis relatively to each other and are combined so that the new resultant characteristic exhibits exactly the same voltage for any condition of receiver tuning. These separated portions of the original characteristic are represented by the curves of Figs. 2B and 2C, and the resultant combined characteristic is shown in Fig.4 2D, and having a pair of spaced parallel linear portions forming a saw-tooth characteristic. Having obtained a characteristic such as that of Fig. 2D, it will be seen that the mark frequency Fm and the space frequency F5 will each increase or decrease by exactly the same amount, the only requirement being that one of the signals e. g., F. be higher than the frequency Fn, and the other signal Fm be lower in frequency than Fo.

If these equal resultant voltages are applied to a linear amplifier tube and the tube plate current is passed through a suitably calibrated meter, the deflections of the meter pointer will be proportional to the applied voltages. If the meter scale is calibrated in frequency units, the amount of detuning of the received signal can be read on the meter directly in terms of frequency. The current resulting from Et which represents the in-tune voltage, should cause half scale meter deflection, and this half scale deflection can be calibrated as zero frequency.

The meter will then directly indicate receiver detuning in either direction.

Fig. 1 shows in schematic form a simplified manner of achieving the above noted results. The signals from the balanced discriminator 4 are applied to two similar diode rectiers 5, 6, in which the above-mentioned separation of the positive and negative portions of the discriminator output is effected. The negative or space voltage is rectied by diode 5 and appears at point 1, while the positive or mark voltage is rectified by diode 6 and appears at point 8. The negative or space signal is applied over conductor 9. thence via the right hand fixed contact I0 and shiftable armature II of an electromagnetic relay I 2 to the control grid I3 of a linear amplifier tube I4 whose plate circuit includes the operating winding of any well-known indicating meter I5. 'I'he armature II is normally closed on contact I0. The deflection of the meter pointer will therefore be proportional to the rectified negative portion of the discriminator output voltage. On the other hand when the discriminator delivers a positive or mark voltage, the diode 5 is non-conductive and the diode 6 is conductive. This results in the application of a suiiicient positive voltage to the control grid of amplifier tube I6 to cause a sufficient plate current to flow through the operating winding I1 of relay I2 to move the armature II into engagement with the other fixed contact I8. The positive rectied mark voltage at point 8 is then applied in series with the opposing battery I9 to the grid I3 of tube I4. 'I'he Voltage of battery I9 is chosen to produce the same resultant voltage' at grid I3 as was previously produced by the space voltage over conductor 9. Eifectively therefore, the positive portion of the discriminator characteristic (Fig. 2A) has been shifted negatively so that mark and space signals produce the same voltage Et (Fig. 2D).

Instead of using a xed battery bias and an electromagnetic relay to control the shifting of one portion of the discriminator characteristic, the alternative electronic switching arrangement shown in Fig. 2 may be employed. In Fig. 2, the parts which correspond to, and function the same as those of Fig. l, are represented by the same numerals. Thus the separation of the positive and negative portions of the dlscriminator characteristic (Fig. 2A) is accomplished by diode rectiers 5 and 6. The positive or mark voltage developed at point 8 is applied to the control grid 2I of a triode 22. The negative or space voltage developed at point 1 is applied to the control grid 23 of another triode 24 similar to triode 22. If desired, the tubes 22 and 24 may take the form of any well-known dual triode. The space voltage developed at point 1 is also applied to any well-known D. C. limiting amplifier comprising, for example, the D. C. amplifier tubes 25, 26. In the amplifier tubes 25, 26, the negative potential representing the space signal is limited when it reaches approximately one fth of the total amplitude which appears at point 1. Furthermore, the output voltage of tube 26 is zero when no signal voltage is applied to the grid of tube 25. When the receiver 3 is in tune and Fo (Fig. 2D) represents zero voltage output from the discriminator, the space signal develops a negative voltage at point 1. The amplifier 25, 26, must be adjusted to deliver twice this negative voltage at its output. This negative voltage is then applied to the cathode 21 of triode 24, where it effects a positive voltage shift of the negative portion of the discriminator characteristic.

Consequently, under mark and space keying conditions, each of the triodes 22 and 24 will draw equal current and the combined plate current as indicated by meter I5 will be a direct measure of the correctness of the receiver tuning under any keying conditions.

While certain specific embodiments have been disclosed, various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In combination, a frequency discriminator o1' the type having a frequency versus voltage characteristic with a single linear sloped portion between successive positive and negative peaked wave portions, a pair of rectiiiers connected to the r FMP output of said dlscriminator, and electron tube means connected to said rectifiers to effectively shift one portion of said characteristic along the voltage axis to provide a resultant characteristic which has a saw-toothed working portion.

2. In combination, means to receive frequency shift signals, a frequency discriminator of the balanced type, a pair of oppositely poled rectifers connected to the output of said discriminator for respectively rectifying the frequencyK shift signals on opposite sides of a mean frequency, a gridcontrolled amplifier tube having its control grid biased under control of one rectifier, another gridcontrolled tube having its control grid biased under control of the other rectifier, means controlled by the current of the other rectier to develop a limited and amplified bias voltage for the cathode of the other amplier tube for effectively shifting the applied grid voltage to said other amplier tube along its grid voltage-plate current characteristic in order to render its plate current equal to the plate current of the rst grid-controlled amplifier tube, and a common load circuit for both said amplifier tubes, the characteristic which relates voltage amplitude in said load circuit to the frequency of said signals having a saw-tooth wave portion with the parallel inclined portions thereof disposed equally on opposite sides of a point representing the said mean frequency.

3. In combination, a frequency discriminator of the balanced type having a frequency versus potential characteristic which has positive and negative wave portions symmetrical with respect to a predetermined mean frequency, oppositely poled rectifier means connected to the output of said discriminator to rectify the signals therefrom,

and an electron tube network connected to said rectiers to displace one of said rectified portions along the voltage axis and thereby to produce a resultant frequency versus voltage amplitude characteristic having a saw-toothed section with a pair of linear sloped portions symmetrically displaced on opposite sides of said mean frequency.

4. In combination, means to produce a voltage of one frequency representing a telegraph mark signal, means to produce a voltage of a different frequency representing a telegraph space signal, a balanced discriminator upon which said two frequencies are impressed, means to rectify separately the mark frequency and the space frequency in the output of said discriminator, a single tuning indicator device, and means connecting said device to said rectiflers so that if the mean frequency between said mark and space frequencies varies from a predetermined value, the resultant voltages representing respectively the mark and the space signals, preserve their original ratio and are of the same relative polarity.

5. In a system for receiving a frequency-shifted carrier having a carrier frequency Fm representing a mark signal and a carrier frequency Fs representing a space signal, a receiver tuned to the mean carrier frequency Fo, a balanced discriminator connected to said receiver and having a frequency versus potential characteristic with a single linear sloped portion joining successive positive and negative peaked wave portions, a first rectifier for rectifying the mark potentials from said discriminator, a second rectifier for rectifying the space potentials from said discriminator, a first grid-controlled tube responsive to the rectified mark potentials, a second grid-controlled tube responsive to the rectified space potentials, and a single tuning indicator device controlled jointly by both said tubes for producing an indication lrepresenting directly the amount and direction of detuning of said receiver with respect to said mean frequency Fn, the characteristic which relates voltage amplitude applied to said tuning indicator with respect to the frequency of said carrier, has a sawtooth portion with the inclined parallel sections thereof equally spaced on opposite sides of a point representing the said mean carrier frequency.

6. In a telegraph system for receiving a carrier of frequency Fm representing a mark signal and another carrier of frequency Fs representing a space signal, a receiver for said carriers and normally tuned to the mean frequency therebetween, and means to indicate when said receiver is detuned and the direction of detuning, the lastmentioned means including a. discriminator having a frequency versus potential characteristic with a single linearly sloped portion representing the range of shift between Fm and Fs, means to rectify separately the mark signal voltage and the space signal voltage from said discriminator, a tuning indicator device, a grid-controlled tube whose plate current controls said device, a switch for normally applying the rectified space signal to thegrid of said tube, means controlled by the rectified mark signal for operating said switch to apply to said grid a voltage which is the difference between the rectified mark voltage and a fixed voltage so that when the receiver is detuned an indication is produced on said indicator representing the amount of detuning and the frequency deviation thereof, the characteristic which relates voltage amplitude applied to said indicator with respect to the frequency of said carrier, has a saw-tooth portion with the inclined -parallel sections thereof equally spaced on opposite sides of a point representing said mean carrier frequency.

7. A telegraph system according to claim 6 in which said switch is in the form of an electromagnetic relay, and another grid-controlled tube has its control grid connected to the mark signal rectifier and its plate circuit includes the winding of said relay.

8. In a telegraph system-for receiving a carrier frequency Fm representing a telegraph mark signal, and a carrier frequency Fs representing a telegraph space signal, a receiver for said carriers `and normally tunedA to produce a predetermined base potential representing said mean frequency, and means to indicate when said receiver is detuned, the last-mentioned means comprising a balanced discriminator, a pair of rectiers connected to said discriminator to rectify separately the mark and space signals therefrom, a pair of grid-controlled amplier tubes, a tuning indicator device connected in balanced relation to the plate circuits of said tubes, said tubes producing equal effects on said indicator when said receiver is properly tuned, means to control the plate current of one of said tubes by the rectified mark signal. means to control the plate current of the other of said tubes by the rectied space signal, and means to proportion the voltages applied to the grids of said tubes so that when said receiver is detuned the plate currents of said tubes vary equally and in the same potential sense for mark and space signals.

9. In a telegraph system, means' to generate a mark frequency Fm, means to generate a space frequency Fs. receiver means to produce a predetermined base potential representing the mean between said mark and space frequencies, a balanced discriminator for said mark and space frequencies, a rectier network connected to said discriminator for respectively rectifying the positive and negative portions of the voltages from said discriminator on opposite sides of the mean frequency between Fm and Fs, an electron tube network connected to said rectifier network, a common load circuit for said electron tube network, the characteristic which relates voltage amplitude in said load circuit to frequency of signals applied to said discriminator being substantially saw-tooth with the parallel inclined portions of the saw-tooth characteristic being located symmetrically on opposite sides of a point representing said mean frequency.

10. A. frequency discriminator system for an applied frequency spectrum having frequency excursions on opposite sides of a mean or center frequency, and comprising means to convert said frequency spectrum into a Variable amplitude spectrum wherein the characteristic curve relating voltage amplitude to frequency characteristic is of saw-tooth Wave form With the inclined portions of the saw-tooth having the same parallel slope and equally disposed on opposite sides of a point representing said mean center frequency, the last-mentioned means including a pair of oppositely poled rectiiiers for rectifying the frequencies of the applied spectrum, and a pair of grid-controlled tubes having a common output circuit and with their .control grids connected respectively to the cathode and anode of said rectiiiers.

1l. In combination, a frequency discriminator of the type having a frequency-versus-voltage characteristic wherein the relation between input frequency and output voltage in the range between two predetermined frequencies has a single-A linear sloped portion, and means connected to the output of said discriminator to convert said single linear sloped portion into two parallel sloped portions displaced along the frequency axis, the last-mentioned means comprising rectiflers for respectively rectifying signals represented by the portions of said characteristic above a predetermined mean voltage and signals represented by the portion of said characteristic below said predetermined mean voltage, and grid-controlled electron tubes having their control grids connected respectively to the cathode and to the anode of said rectiers said electron tubes having a common output circuit wherein the relation between frequency and voltage is characterized by two parallel sloped portions displaced along the frequency axis.

12. The combination according to claim 11, in which said frequency discriminator is of the balanced type.

REYNOLD S. CHAPIN.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 2,211,750 Humby Aug. 20, 1940 2,309,481 Summerhayes Jan. 26, 1943 2,333,990 Dome Nov. 9, 1943 2,387,496 Cornelius Oct. 23, 1945 2,412,482 Vilkomerson Dec. 10, 1946 2,423,616 Rath July 8, 1947 2,423,617 Rath July 8, 1947 

